1. Field of the Invention
The present invention relates to golf ball cover compositions which have a good moldability and are capable of providing golf ball covers of excellent resilience, durability and feel upon impact. The invention also relates to golf balls which have a cover made of such a composition and are thereby endowed with excellent rebound characteristics, durability and feel upon impact, as well as improved distance.
2. Prior Art
Ionomer resins are widely used today as a cover material in golf balls. These resins are typically made of an ionic copolymer of an olefin such as ethylene with an unsaturated carboxylic acid (e.g., acrylic acid, methacrylic acid, maleic acid), in which a certain proportion of the acidic groups have been neutralized with metal ions such as sodium or zinc. Such resins have outstanding characteristics such as durability and resilience which make them highly desirable as the base resin in golf ball cover stock.
Using a relatively hard ionomer resin as the golf ball cover stock serves to reduce friction between the club face and the ball during a driver shot, thus holding down backspin when the shot is taken. As a result, the ball is less affected by the wind when aloft and rolls well after landing, making it possible to achieve a good overall distance.
However, such ionomers have one significant drawback when used as the cover material in golf balls: they give the ball a very hard feel at the moment of impact.
To overcome this problem, U.S. Pat. No. 4,884,814 and JP-A 1-308577 disclose the use of a golf ball cover made of a blend of soft and hard ionomer resins, and specifically one arrived at by blending a given amount of a relatively soft ethylene/(meth)acrylic acid/(meth)acrylate terpolymer ionomer resin with an ethylene/(meth)acrylate copolymer ionomer resin of a hardness within a certain range.
This prior art has indeed provided a very effective way to significantly improve the hard feel of golf balls having a cover made of an ethylene/(meth)acrylic acid copolymer type ionomer resin.
However, golf ball covers made of a soft and hard ionomer resin blend as taught by the foregoing art have a number of drawbacks. To begin with, the relatively soft ethylene/(meth)acrylic acid/(meth)acrylate terpolymer type ionomer resin has a low resilience and a low mechanical strength, which lowers the rebound characteristics, total distance traveled and durability of golf balls made therewith. Moreover, ethylene/(meth)acrylic acid/(meth)acrylate terpolymer type ionomer resins have a high viscosity, which undesirably increases the viscosity of the resin blend, detracts from its moldability, and tends to result in a ball having a poor sphericity.
U.S. Pat. No. 4,674,751 describes a golf ball in which the cover is made of a resin having a specific hardness and prepared by melt blending under the application of heat an ionomer resin of a specific hardness with a thermoplastic urethane resin of a specific hardness. Unfortunately, because the ionomer resin and the thermoplastic urethane resin have a poor compatibility, the cover material thus disclosed has a poor moldability and molded covers made therewith have a poor durability to repeated impact.